This invention relates generally to single-stage end-suction centrifugal pumps and more particularly to centrifugal pumps with both open and shrouded impellers for low-flow, high head applications.
Centrifugal end-suction pumps are well known and are in wide use. Many different types of such pumps are available, but not many are specifically designed for low flow rates where a high head is desired, along with good efficiency, good suction performance, and high pump reliability (or low maintenance). In most cases, a low-flow duty is met with a pump sized for more flow than is required by the intended application. This provides the required pumping capacity, but it means the pump has to operate off design where not only is energy wasted, but the potential for damage is increased because of highly unsteady hydraulic loads due to internal flow separation. Furthermore, the generation of high head at low flow is more difficult, since a high head coefficient must be achieved in order to maximize head for a given impeller diameter while maintaining reasonable hydraulic load levels for both steady and unsteady components of radial and axial forces.
The most common pump design has an impeller with a narrow width and a low number of vanes, which leads to a large diameter impeller and a large size/high weight pump. The suction performance in relation to cavitation is only fair.
Some special pumps designed for this duty have a narrow small diameter discharge casing with a correspondingly narrow, multi-vane, optimized-diameter impeller. Multivane impellers for low-flow operation generally do not have inlet conditions suitable for operation at low local suction pressure. This is due to the poor matching of blade angle to flow angle and the blockage (or occlusion) of the inlet caused by the vanes themselves. As a consequence of this, the potential for poor cavitation behavior is increased, which invites several negative effects, namely: a) the pump produces pronounced decay of head and efficiency unless high suction pressure is provided by highly elevating the feed tank (which increases installation cost of the tank), or by reducing the pump motor speed; b) the the pump is subjected to highly unsteady flow, even surge, because of pressure pulsations induced by large vapor volumes inside the pump, thereby reducing pump reliability and increasing maintenance costs; and c) the impeller can be quickly damaged by cavitation erosion along with other pump components, such as the wear ring, suction vanes, volute tongue, or diffuser vanes.
Cavitation, which contributes to damage and loss of efficiency, is caused by the hydraulic pressure head at the impeller inlet falling below the vapor pressure of the working fluid. This results in formation of bubbles and their subsequent collapse at the surface of the impeller. Collapse of millions of such bubbles, each producing a micro-shock, locally erodes the impeller surface and ultimately causes pitting, perforation, and failure of the impeller.
It is highly desirable for a pump, which needs to operate with small capacity and high head, to have a design capacity close to the operating capacity in order to minimize all the negative effects related to off-design operation. Such a pump should be optimized for low flow coefficient, high head coefficient, high efficiency, and low net positive suction head (NPSH). This suggests use of a small impeller diameter and a large number of vanes with a steep blade angle and narrow width at the exit of the impeller, along with low blade blockage (a low number of vanes) and a small blade angle at the inlet.
The foregoing illustrates limitations known to exist in present centrifugal pumps. Thus, it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.